Commonly, alkali metal halide brines, such as sodium chloride brines and potassium chloride brines, are electrolyzed in an electrolytic cell wherein a liquid permeable diaphragm divides the cell into an anolyte compartment with an anode therein and a catholyte compartment with a cathode therein to produce chlorine, hydrogen, and aqueous alkali metal hydroxide. Asbestos has been the most common diaphragm material, but has suffered from relatively short lifetimes and from environmental concerns. Asbestos-free microporous diaphragms have been produced by sintering materials such as polytetrafluoroethylene (PTFE) and a particulate pore forming additive followed by subsequent removal of the additive, as shown by for example U.S. Pat. Nos. 3,930,979, 4,098,672 and 4,250,002. While such microporous diaphragms have a long service life, they have been produced in the form of sheets and are not easily utilized in electrolytic cells having complex nonplanar electrode geometries, such as diaphragm cells with fingered anodes and cathodes as shown in U.S. Pat. No. 3,910,827. Additionally, the temperatures required to sinter commercial size diaphragms of a material such as polytetrafluoroethylene result in a significant energy consumption.
U.S. Pat. No. 4,036,729 describes depositing discrete thermoplastic fibers of, e.g., a fluorinated hydrocarbon, from an aqueous medium containing acetone and preferably a fluorocarbon surfactant onto a cathode screen for use as a diaphragm in electrolytic cells. The deposited fibers form an entanglement or network which does not require bonding or cementing. Unfortunately, such polyfluorocarbon diaphragms generally are hydrophobic, i.e., difficult to wet with water. This hinders dispersion of the polyfluorocarbon fibers in an aqueous medium prior to deposition, hinders passage of an aqueous electrolyte through the diaphragm, and results in high cell voltages, particularly in comparison to asbestos-based diaphragms under similar cell conditions.
U.S. Pat. No. 4,482,441 describes codeposition of fibrils of a hydrophobic organic polymer, e.g., a copolymer of tetrafluoroethylene and perfluoropropylene, and a hydrophilic group IIA metallic oxide, e.g., magnesium oxide particles, from an alkaline brine containing sodium hydroxide, sodium chloride and a polyethyleneimine-based retention agent onto the cathode of a cell. Such a deposited diaphragm may also include a surface active agent, e.g., a fluorinated surface active agent.
U.S. Pat. No. 4,170,539 describes a diaphragm having a porous, hydrophobic fluorocarbon matrix, an intermediate layer or film of a hydrophilic fluorocarbon resin, e.g., a perfluorinated polymer having pendant ion exchange groups, on the surfaces of the matrix, a hydrous oxide of zirconium and optionally a hydrous oxide of magnesium, the hydrous oxides contained within the pores of the matrix. The intermediate layer of hydrophilic resin is applied onto pre-formed hydrophobic matrix.
Finally, U.S. Pat. No. 4,606,805 describes a diaphragm containing as its principal particulate ingredient an inorganic material such as talc, a metal silicate, an alkali metal titanate, an alkali metal zirconate or a magnesium aluminate, along with both polytetrafluoroethylene fibers and polytetrafluoroethylene particulates. After deposition, such a diaphragm is sintered at temperatures of about 350.degree. C.
It is desirable to have diaphragms, e.g., polyfluorocarbon-based diaphragms, prepared by depositing the diaphragm material onto the cathode of a cell, the deposition preferably being from an aqueous slurry.